Figure 3

Putative gene function in ascorbate homeostasis and Al tolerance. Al tolerance-associated genes are represented by white numbers* in the dark blue boxes (numbers in black are other genes of the same type associated with Al stress). Genes associated with Al stress responses are in black boxes. Box 1) The normal process of cell wall loosening is promoted by the production of hydroxyl radicals (^•OH) to break glycosidic linkages (sugar-sugar). The hydroxyl radicals are produced non-enzymatically by the Fenton chemistry in the presence of Cu²⁺, H₂O₂ and ascorbate which serves as electron donor (the three molecules are in grey boxes). In the apoplasm, ascorbate (ASC) is transformed to monodehydroascorbate (MDHA + e^-) and a free electron is used to reduce the Cu²⁺ to Cu⁺ (needed for the Fenton reaction). An ascorbate oxido-reductase (ASC ox-red), or other plasma membrane transporters may be involved in the regeneration of apoplasmic ASC. Cytoplasmic ASC and NADH are needed to maintain the level of ASC in both compartments. Al (red box) was shown to block a redox reaction (electron transfer) [34] suggesting that it interferes either with the electron transfer from the cytoplasm to ASC (regeneration) or from ascorbate to Cu²⁺ (utilization). The diverted electron may generate different ROS molecules that will cause oxidative stress and trigger a redox response and the induction of various genes. Box 2) A slower regeneration of ASC could lead to its degradation and to the accumulation of oxalate. The induction of oxalate oxidase can use this oxalate to maintain the production of H₂O₂ and support the Fenton reaction. If the apoplasmic ASC concentration decreases too much, H₂O₂ may not be used efficiently in the cell wall loosening process and will accumulate. This ROS can cause the oxidation of several targets including different cysteine-rich proteins associated with Al tolerance. GSTs associated with Al stress or tolerance can participate in the protection or repair of oxidized targets. Box 3) Mobilisation of phloem sugars and reduction in phosphate availability (Low Pi in green) caused by Al can also participate in gene regulation and stimulate alternative pathways to increase the availability of important metabolite precursors such as NADPH (in light grey) through the pentose-P pathway (G6P: Glucose-6 phosphate; G6PDH: glucose 6 phosphate dehydrogenase); and phosphate (F1,6BP, Fructose 1,6 bisphosphatase; PPi PFK: pyrophosphosphate dependent phosphofructokinase). Other alternative pathways are activated to maintain the Krebs cycle (PDH: pyruvate dehydrogenase, AOX: alternative oxidase) and to stimulate malate production (PEPC: phosphoenol pyruvate carboxylase; MDH: malate dehydrogenase). UDP-glucose can be used to generate ASC and NADH (arabinonolactone oxidase). Other genes such as ALMT1 or ABC transporters may be involved in Al chelation and transport to exclude Al. *: the numbers represent genes identified in this work; see Table 1 and Table 2. o: genes, enzyme activities or metabolite changes identified in other studies. For references, see text.